Abstract P2-11-09: Excellent acute toxicity outcomes with proton therapy for partial breast irradiation in early stage breast cancer: Initial results of a multi-institutional phase II trial

Background and Purpose: Partial breast irradiation (PBI) with proton therapy after lumpectomy for early stage invasive breast cancer is an area of active investigation. Advantages of this technique include a shorter treatment course and the potential for decreased morbidity versus external beam photon radiation therapy given superior sparing of the surrounding normal breast tissue. To date, multiple single-institutional studies have reported conflicting results on the acute toxicity of PBI. This prospective phase II trial investigates the feasibility, safety, and efficacy of delivering PBI with proton therapy in a multi-institutional setting. Methods: Patients over the age of 50 years with ER-positive nonlobular invasive breast cancer or ductal carcinoma in situ ≤3 cm in size who had undergone lumpectomy with at least 2 mm negative surgical margins were treated with proton therapy to a dose of 40 Gy delivered over 10 daily fractions. In this initial analysis, we assess early toxicity and treatment efficacy of proton PBI. Patients were followed up at 4 weeks post-treatment and annually thereafter, along with annual mammograms. Patient-reported quality of life and physician-reported cosmesis assessments including photographs were obtained at 1 and 3 years post-treatment. Results: Forty patients were enrolled, of which 38 were evaluable. At a median follow-up of 17.8 months (range 2-36 months), all patients had overall breast cosmesis that was scored “good” or “excellent”. Of 6 grade 2 acute adverse events that occurred, only 1 was radiation dermatitis, with others including lymphedema, hot flashes, and fatigue. One grade 3 acute toxicity occurred 3 weeks after radiation completion in the form of vascular disease requiring stent placement, highly unlikely to be attributable to radiation effects. Patient-reported quality of life outcomes were recorded using the standardized Breast Cancer Treatment Outcome Scale (BCTOS) scored from 1-4 (1: none; 2: mild; 3: moderate; 4: large), with endpoints receiving a score of 3 or 4 most frequently involving change in breast size, breast texture, nipple appearance, or scar tissue. Patients assigned a score of 4 for change in nipple appearance (n=2), breast shape (n=2), and scar tissue formation (n=2). To date, local, locoregional, and distant disease control are 100%, although one patient has developed a new hormone receptor-negative invasive ductal carcinoma of the contralateral breast. Conclusion: Proton PBI provides excellent early cancer control with acceptable cosmetic outcomes and minimal adverse effects as per patient- and physician-reported assessments. On continued follow-up, late toxicity and cosmesis, as well as long-term disease control outcomes, will be assessed. Citation Format: Choi JI, Chang AL. Excellent acute toxicity outcomes with proton therapy for partial breast irradiation in early stage breast cancer: Initial results of a multi-institutional phase II trial [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-11-09.